Fix QuantityType arithmetic of mixed units

[jimtng]

This PR addressed the following problems:

1. Operates under the rhs unit

a = QuantityType.new("20 °C")
b = QuantityType.new("9 °F")
logger.warn a + b # => 45 °F - this should be 25 °C 
logger.warn b + a # => 25 °C - this should be 45 °F

self gets converted to other's unit + other converted to self.unit then added to self. This gave the wrong result in the wrong unit.

The lhs just needs to be converted to the unit defined by the block, and rhs needs to be converted using to_unit_relative to the unit defined by the block.

2. Inside a unit block

a = QuantityType.new("20 °C")
b = QuantityType.new("9 °F")
unit("°C") do
  logger.warn a + b # => 7.2222 °C => This should be 25 °C
  logger.warn b + a # => 7.2222 °C => correct. 9 °F + 20 °C -> -12.78 °C + 20 °C
  logger.warn 2 + b # -> - 20.999 °F => Should be 2°C + 5°C = 7 °C
  logger.warn 2 - b # => 24.999 °F => Should be 2 - 5 = -3 °C
end

3. Multiplications inside a unit block didn't take up the block's unit

kw = 5 | "kW"
unit("W") do
  logger.warn (kw * 2).format("%.0f %unit%") # => 10 kW => Should turn into 10000 W
  logger.warn (2 * kw).format("%.0f %unit%") # => 10 kW => Should turn into 10000 W
end

4. +/- against non-quantity type is still allowed. It takes up the other operand's unit. This should raise an exception instead.

kw = 5 | "kW"
logger.warn kw + 5 # => 10kW
logger.warn 5 + kw # => java.lang.NullPointerException: Cannot read field "quantity" because "state" is null

5. Fix failing specs due to core changes in Adjust QuantityType calculations for temperatures openhab-core#3792

[jimtng]

PS the last 2 commits are a bit intermixed, so perhaps just squash-merge them.